Solar energy collection systems in the prior art include those involving unfocused solar energy and those including focused solar energy. In the case of unfocused solar energy it will be appreciated that while it is theoretically possible to obtain 10,300.degree. F. from a focused spot it is only theoretically possible to obtain 660.degree. F. in the unfocused case. In practical systems using unfocused radiation and flat plates to heat water, only 100.degree. F.-145.degree. F. is possible. The result is that systems utilizing unfocused solar energy, e.g. flat plate systems, are only 10-20% efficient in converting solar energy to do work as compared with an 80% efficiency obtained when solar energy is focused. This is because in heat engines, the higher the temperature of the input fluid the more efficient will be the conversion of thermal energy into mechanical energy. Up to this time, focused solar energy has been coupled into a liquid such as water for producing steam which is used to do work. Because liquids at high temperatures are extremely corrosive, handling problems prevent the use of most systems which utilize liquid as the transfer medium to maximum efficiency. For example, almost all of the systems utilizing focused solar radiation have up to now been based on the conversion of water into steam in a boiler. These systems typically require a continuous water supply for cooling and are usually limited to temperatures below 1000.degree. F. Above this temperature, steam becomes increasingly corrosive, severely limiting the useful life of many of the components in the system. Because of these difficulties the efficiency of a steam cycle plant is limited. It should be noted that systems involving liquid-gas phase transitions, like steam, are generally optimal over a narrow range of pressures and temperatures. Moreover, if the critical temperature of the fluid is exceeded there will be no transition from the vapor phase to the liquid phase, thus eliminating the low power advantage associated with moving liquids as opposed to vapor.
It has been found, however, that a system in which solar energy is coupled into a gas presents fewer problems in the handling and is operable over a much higher and wider range of temperatures. Thus, the transfer medium in the subject system is gas. One such system is illustrated in French Pat. No. 1,035,833 issued in 1953 to Felix Trombe. In this system solar energy is focused onto a solar receiver which has a restricted aperture and a dome like window so that heat from the heat exchanger in the receiver will not melt the window. In this patent the window is prevented from melting, not by impingement cooling, which is the subject of the present invention, but rather by restricting radiation from the heat exchanger and curving the window to be as far removed from the heat exchanger as possible. This present system utilizes a receiver which can withstand the high temperatures made possible by focusing solar radiation and is characterized in one embodiment by a sealed chamber which has a honeycomb heat exchanger positioned within the chamber with its tube-like channels parallel to the axis of the focusing optics. The purpose of the honeycomb structure is to provide a large wall area for the gas to contact, with the walls running parallel to the axis of the focusing optics so that the focused light impinges on the honeycomb structure "end on". This "end on" configuration is used so that all of the impinging radiation is internally reflected within the honeycomb until it is absorbed by the walls, such that very little solar energy is reflected back to the focusing optics.
In an important aspect of the subject invention, one end of the chamber is sealed with an impingement cooled window to prevent escape of energy from the heated honeybomb. The impingement cooling prevents the window from melting and thus permits the receiver to operate at very high temperatures in a closed or sealed system. The receiver in this configuration is so efficient it acts as a black body receiver to collect large amounts of solar energy. The receiver operates at much higher temperatures than steam systems or liquid systems in general and the overall efficiency of the conversion system is enhanced to the extent that higher temperatures result in higher conversion efficiencies. For the present purposes, a black body receiver is a device which absorbs all or nearly all of the energy impinging on it for wavelengths of interest. The absorption of this energy results in the raising of the temperature of the body, in this case the specialized honeycomb heat exchanger within the receiver. For purposes of this invention the range of wavelengths in the solar energy spectrum which are most effective to heat up the heat exchanger are between 0.3 and 3.0 microns.
Obviously it is impossible to make a perfect black body receiver. However, in order to efficiently transfer solar energy to a gas, a new type of heat exchanging system was developed in which retroreflection of the focused energy is limited both by the "end on" honeycomb illumination and by one or more windows which are transparent to short wavelength solar energy, while reflecting energy produced by long wavelength radiation from the heated honeycomb. In the embodiments discussed, the windows are impingement cooled in a manner which enables them to withstand the extremely high temperatures involved in the focusing of solar energy. A less efficient system, also within the scope of this invention, involves dispensing with the windows and merely sucking air through an unrestricted aperture past the honeycomb heated by focused solar radiation.
By using a solar-gas system, and by making the honeycomb of silicon carbide and using impingement cooled quartz windows, the apparatus operates at temperature well above 2000.degree. F. and as high as 5000.degree. F. Moreover, since ordinary air from the environment can be used as the heat transfer fluid, cooling can be achieved by allowing spent gas to escape to the atmosphere if an open cycle system is desired, eliminating the need for access to a large volume of water. If a closed cycle system is preferred, the gas can be nitrogen or other relatively inert inexpensive gas as well as air.
What is therefore provided by this invention is a black body receiver which includes an enclosed chamber and a window through which solar radiation is focused. The window is cooled to prevent melting by the influx of the cool gas which is to be heated by the exchange of energy between the solar radiation and the gas. The cool gas impinges upon the window and is redirected in a turbulent fashion towards a honeycomb heat exchanger in which the honeycomb channels are illuminated "end on" and are heated by the solar energy. The honeycomb readily absorbs solar energy and heats up regardless of whether the honeycomb walls are primarily specular of diffuse because of the internal energy reflection within the channels. The channels are long enough so that substantially all the energy impinging on the heat exchanger is absorbed via multiple internal reflections in which some energy is absorbed with each reflection. Thus, nearly all incident energy is finally absorbed. The honeycomb is not only an efficient light energy absorber, but its channeled construction also makes it effective for heat transfer by convection to a gas flowing through it. The turbulent gas is forced through the honeycomb and obtains energy by contact with the heated honeycomb walls.
In one embodiment multiple windows are utilized with the outside window being transparent to the incoming radiation while at the same time being able to contain the incoming gas. The windows prevent radiation from the heated honeycomb from escaping because they are reflective in the red and infrared region of the electromagnet spectrum, and therefore reflect this radiation back towards the honeycomb. In between the outer window and the honeycomb structure a number of windows may be provided which are apertured to increase the turbulent flow arond the windows while permitting the gas to impinge upon and flow through the honeycomb structure. The gas turbulence results in cooling of both the outer and inner windows while secondarily the gas absorbs heat from the windows. The gas flow pattern throughout the heat exchanger is such that the largest portion of gas is directed towards the honeycomb structure after first impinging on the outer window, while secondary turbulent gas is permitted by virtue of the window configuration and by the window mounting apparatus which permits the flow of gas around the inner windows thereby to establish primary and secondary flow patterns to optimize the heat exchange.
In one embodiment the subject heat exchanger may be mounted on a mast above an array of individually steerable reflective elements which track the sun and focus the sun's radiation onto the heat exchanger.
It is therefore an object of this invention to provide an improved solar energy conversion system in which focused solar energy is coupled into a heat transfer medium which is a gas rather than a liquid.
It is another object of this invention to utilize focused solar energy and a high temperature energy converter for maximum efficiency in solar energy conversion, in which the energy converter includes a heat exchanger having channels illuminated "end on" by focused solar radiation to minimize retroreflection.
It is a still further object of this invention to provide a solar energy conversion system in which the heat exchanger operates at an extremely high temperature and includes an enclosed chamber with a transparent wavelength selective window which permits the influx of solar radiation while preventing the thermal energy radiated from the heat exchanger from escaping.
It is yet another object of this invention to provide a high temperature black body receiver utilizing an enclosed chamber and multiple windows, along with a honeycomb heat exchanging member in which a primary and secondary flow path of gas is provided with the primary flow directly into the honeycomb structure and with the secondary flow involving a turbulent gas flow around the windows for cooling purposes.
It is a still further object of this invention to provide an inexpensive solar energy conversion system in which a solar-to-gas cycle is utilized.
These and other objects of this invention will be better understood in connection with the following description in view of the appended drawings in which: